Various types of optical amplifiers, such as erbium-doped fiber amplifiers (EDFAs) and distributed Raman amplifiers (DRAs), are ubiquitous components of optical communication systems, eliminating the need to perform optical-electrical-optical signal transformations when regeneration of a fading optical signal is required.
In the case of EDFAs, an optical pump laser (typically operating at 980 nm) is coupled into a section of Er-doped optical fiber, and the incoming optical signal is co-propagated through the doped fiber with the pump light. The presence of the pump light with the erbium dopant generates amplification of the propagating optical signal by the transitions of the optically-excited erbium ions. In addition to the pump source and the doped fiber (as well as the optical couplers required to inject the signal and pump light into the fiber), a conventional optical amplifier module includes a filter component (typically a WDM component) that is used to introduce the signal and pump into the fiber. The incoming optical signal also needs to be isolated from back reflections along the input path, necessitating the use of an optical isolator along the input signal path. Isolation is also necessary at the output of the amplifier, to prevent high power optical output signals from being reflected back into erbium-doped fiber itself.
The optical gain may vary spectrally, which may create a non-uniform amplification of the various wavelengths within the amplifying fiber. To improve uniformity in the amount of amplification provided at each different wavelength forming the input signal, a gain-flattening filter may be included, and positioned to receive the amplified output from the doped fiber.
In certain system applications, it may be necessary to monitor the input and output signals associated with the amplifier, thus providing closed-loop control of the amplifier's performance. Another component that may be required is a variable optical attenuator, which is used to introduce post-amplification attenuation for controlling the power delivered by the output signal. A tunable optical filter is another component which may be included in an optical amplifier to reduce the amount of broadband optical noise (amplified spontaneous emission, or ASE), which is generated during amplification along the span of doped fiber from reaching the output of the amplifier. While a distributed Raman amplifier (DRA) does not utilize rare earth doped fiber to create gain, the Raman amplification process still requires the coupling of an additional light beam (pulses) into an optical fiber and utilizes post-amplification techniques to improve the quality of the amplified optical signal.
The various components forming an optical amplifier module are typically made as fiber-coupled elements, and in some cases integrated (or hybridized) to form, for example, a combined isolator and WDM filter, or a combined isolator and GFF filter, or the like. Of course, lower cost and smaller-sized modules lowers the overall system costs. Thus, the trend to smaller components, more hybridization and smaller modules has been taking place for some time. Indeed, the pressure for smaller form factors and lower costs continues to be exerted on the industry.
One path to assuage these demands is to continually reduce the size of the various components and, perhaps, increase their degree of integration. However, this is not easily accomplished in an environment where the cost of the amplifier module is also a concern. Indeed, the size of these components has decreased to the point where they cannot be readily assembled using conventional industry techniques such as, for example, manual packaging (with the assistance of micrometers) by assembly-line personnel. Indeed, as the level of integration increases and the size of the components decreases (e.g., the size of some of these components can be on the order of 1 mm×1 mm×1 mm), it becomes difficult to have a highly repeatable assembly process with high yield. Moreover, in contrast to electronic integrated circuits, modules such as optical amplifiers also require alignment of the optical beams and creation of a large number of optical splices. All of these issues add yet another level of concern (and cost) to the efficiency of production and the integrity of the final product.
Furthermore, even with reduction in size of an optical amplifier module, such as from increasing the level of integration within the hybrid components, the different hybrids must be coupled to each other via fiber splicing and routing. The fiber splices themselves require splice protectors, which further adds to the size of the assembly (and to the labor-intensive assembly of the module). These fibers also need to be routed between the various components, which may involve the use of yet another element to coordinate the placements and paths that these fibers take (a further impairment to reducing the overall size of the module to meet small form factor device requirements). As a consequence of the minimum bend radius of the optical fiber (i.e., the optical signal loss increases with a smaller bend radius; the physical failure of the fiber increases with a smaller bend radius as well) as well as the relatively large number of fiber splices and splice protectors mandating the same, the ability to further hybridize current configurations is quickly reaching its technical limits, size limits and economical possibilities of implementation.
Thus, for an optical amplifier module to continue to meet the expectations of cost and size reduction, while maintaining performance requirements, a different approach to configuring an optical amplifier module is required.